U.S. patent application number 12/349063 was filed with the patent office on 2009-09-03 for method and device for secure monitoring of a distance.
Invention is credited to Martin HECHINGER.
Application Number | 20090219013 12/349063 |
Document ID | / |
Family ID | 38646317 |
Filed Date | 2009-09-03 |
United States Patent
Application |
20090219013 |
Kind Code |
A1 |
HECHINGER; Martin |
September 3, 2009 |
METHOD AND DEVICE FOR SECURE MONITORING OF A DISTANCE
Abstract
The invention relates to a method for securely monitoring a
distance between a metallic part and an inductive proximity sensor,
in particular for monitoring a closed position of a guard door in
an automated system, said method having the steps of: providing an
oscillating circuit, providing a metallic part made of a first
material, driving the oscillating circuit with at least one pulse,
such that a decaying oscillation is generated which is dependent on
a distance between the metallic part and the oscillating circuit
and which has an actual frequency and an actual attenuation, and
signaling whether both the actual frequency undershoots a
predefined maximum frequency and the actual attenuation undershoots
a predefined maximum attenuation. The invention also relates to a
signaling device for monitoring a distance between a metallic part
and an inductive proximity sensor.
Inventors: |
HECHINGER; Martin;
(Herbolzheim, DE) |
Correspondence
Address: |
HARNESS, DICKEY & PIERCE, P.L.C.
P.O. BOX 828
BLOOMFIELD HILLS
MI
48303
US
|
Family ID: |
38646317 |
Appl. No.: |
12/349063 |
Filed: |
January 6, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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PCT/EP2007/005894 |
Jul 3, 2007 |
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12349063 |
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Current U.S.
Class: |
324/207.15 |
Current CPC
Class: |
H03K 17/972 20130101;
H03K 17/954 20130101; H03K 2017/9706 20130101 |
Class at
Publication: |
324/207.15 |
International
Class: |
G01B 7/14 20060101
G01B007/14 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 7, 2006 |
DE |
10 2006 032 226.6 |
Claims
1. A method for securely monitoring a distance between a metallic
part and an inductive proximity sensor, said method having the
following steps of: providing an oscillating circuit, providing a
metallic part made of a first material, driving the oscillating
circuit with at least one pulse, such that a decaying oscillation
is generated which is dependent on a distance between the metallic
part and the oscillating circuit and has an actual frequency and an
actual attenuation, and signaling whether both the actual frequency
undershoots a predefined maximum frequency and the actual
attenuation undershoots a predefined maximum attenuation.
2. The method of claim 1, wherein it is signaled whether the actual
frequency exceeds a predefined minimum frequency and/or the actual
attenuation exceeds a predefined minimum attenuation.
3. The method of claim 1, wherein a data record is provided which
represents a first correlation between the frequency and the
attenuation as the metallic part approaches, and the signaling
indicates whether the actual frequency and the actual attenuation
correspond to the first correlation.
4. The method of claim 3, wherein the data record describes a
distance between the metallic part and the oscillating circuit,
which distance is dependent on the frequency and attenuation, and a
distance value is determined on the basis of the actual frequency
and the actual attenuation.
5. The method of claim 4, wherein it is signaled whether the
distance value exceeds a minimum distance value and/or undershoots
a maximum distance value.
6. The method of claim 3, wherein the data record represents a
second correlation between the frequency and the attenuation when a
metallic part made of a second material, that is different from the
first material, approaches the oscillating circuit, and in that it
is signaled whether the actual frequency and the actual attenuation
correspond to the second correlation.
7. The method of claim 6, wherein it is signaled to which of the
correlations stored in the data record the actual frequency and the
actual attenuation correspond.
8. The method of claim 6, wherein the data record stores at least
one further correlation for a further material, and it is signaled
whether the actual frequency and the actual attenuation correspond
to a correlation from a particular group of correlations.
9. The method of claims 1, wherein the method is carried out twice,
and in that a fault is signaled if there is a difference in the
respective resultant signaling.
10. A signaling device for securely monitoring a distance between a
metallic part and an inductive proximity sensor, said device having
an oscillating circuit, an associated metallic part made of a first
material and a pulse generator for driving the oscillating circuit
with at least one pulse in order to generate a decaying oscillation
which is dependent on a distance between the metallic part and the
oscillating circuit and which has an actual frequency and an actual
attenuation, and also having a signal transmitter which is designed
to signal when both the actual frequency undershoots a predefined
maximum frequency and the actual attenuation undershoots a
predefined maximum attenuation.
11. The signaling device of claim 10, further comprising a memory
which stores a first correlation between the frequency and the
attenuation as the metallic part approaches, and the signal
transmitter is designed to signal whether the actual frequency and
the actual attenuation correspond to the first correlation.
12. The signaling device of claim 10, wherein the signaling device
is of redundant design by means of a further oscillating circuit
and a further signal transmitter and has a comparison unit for
comparing the signals from the signal transmitters and for
signaling a fault if said signals do not match.
Description
CROSS REFERENCES TO RELATED APPLICATIONS
[0001] This application is a continuation of international patent
application PCT/EP2007/005894, filed on Jul. 3, 2007 designating
the U.S., which international patent application has been published
in German language and claims priority from German patent
application DE 10 2006 032 226, filed on Jul. 7, 2006. The entire
contents of these priority applications are incorporated herein by
reference.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to a method for securely
monitoring a distance between a metallic part and an inductive
proximity sensor, in particular for monitoring a closed position of
a guard door in an automated system.
[0003] The invention also relates to a signaling device for
securely monitoring a distance between a metallic part and an
inductive proximity sensor, in particular for monitoring a closed
position of a guard door in an automated system.
[0004] In order to make safe a hazardous area of a system which
operates in automated fashion, for example robots, presses or
conveyor belts which operate in automated fashion, guard doors are
often provided, which doors prevent the operator from coming into
contact with those parts of the system which may be hazardous to
the operator.
[0005] So that the guard door can fulfill its protective function,
before starting the system it has to be verified that the guard
door is properly closed. For this purpose, an inductive proximity
sensor of the type described below may be used which checks whether
the guard door is at a particular position. The system cannot be
switched on as long as the required position of the guard door has
not been reached.
[0006] The inductive proximity sensor operates according to the
following principle. The proximity sensor contains an oscillating
circuit which is caused to oscillate by means of a pulse. The
oscillating circuit has a characteristic decaying oscillation if a
metal to be detected is in the vicinity thereof. If the metal then
approaches the proximity sensor, the attenuation and/or frequency
of the decaying oscillation change(s).
[0007] However, the fact that the metal to be detected must be
known in advance and the proximity sensor must be constructed in an
appropriate manner gives rise to difficulties in this case. If, for
example, the intention is to detect a particular distance both for
aluminum (non-ferromagnetic) and for iron (ferromagnetic), either a
reconfiguration or a very complex device for determining the
material is required. Only after the material is known can the
distance then be determined. Nevertheless, even in the case of a
material detection, which can distinguish between ferromagnetic and
non-ferromagnetic materials, problems remain in the case of metal
alloys which have both ferromagnetic and non-ferromagnetic
components.
[0008] In addition, it has been found in practice that the
operations of opening and closing the guard door, for example, in
order to clamp a workpiece, are found to be a nuisance by some
operators. In the past, it has therefore been the case that
operators have manipulated the safety device by fixing a piece of
metal directly to the proximity sensor. In this case, the system
could be operated even with the guard door open since the sensor
incorrectly detected the piece of metal as a closed guard door.
[0009] In order to counteract such attempts at manipulation, it is
desirable to determine or monitor the distance between a proximity
sensor and a metallic counterpart in such a manner that the system
is also not switched on or is switched off when the distance of the
metal to be detected is too small.
[0010] DE 102 22 186 C1 discloses a safety switch having two
transmitting/receiving elements. In this case, one element is
fitted in a stationary manner, for example, while the other element
is fastened to a guard door. In a first step, the first element
transmits electromagnetic signals to the second element which is in
the form of an LC-oscillating circuit. As a result, the second
element is caused to oscillate and stores a part of the transmitted
quantity of energy which is dependent on its distance from the
first element. The energy stored in this manner is then emitted in
the form of electromagnetic waves at the frequency of the
LC-oscillating circuit of the second element. A part of this energy
which is again dependent on the distance between the two elements
is then transmitted back to the first element. A quantity of energy
which is a measure of the distance between the two elements is
generated in the first element by means of integration over a
predetermined period of time. If the quantity of energy received
differs from an expected quantity of energy, a state is assumed in
which operation of the corresponding system is not allowed.
[0011] EP 1 278 077 A2 discloses the general principle of an
inductive proximity sensor which does not depend on an
appropriately designed counterpart (transmitting/receiving
element). However, the sensor does not have the protection against
manipulation required for use in safety technology.
[0012] Against this background, an object of the present invention
is to show a cost-effective device and a method for securely
monitoring a distance between a metallic part and an inductive
proximity sensor even if the approach of different metals needs to
be taken into account. In this case, the intention is to be able,
in particular, to protect against manipulation in a manner which
ensures compliance with a minimum distance and allows the same
minimum distance for different metals.
SUMMARY OF THE INVENTION
[0013] According to one aspect of the invention, there is provided
a method of the type mentioned at the outset, said method having
the following steps of: [0014] providing an oscillating circuit,
[0015] providing a metallic part made of a first material, [0016]
driving the oscillating circuit with at least one pulse, such that
a decaying oscillation is generated which is dependent on a
distance between the metallic part and the oscillating circuit and
which has an actual frequency and an actual attenuation, and [0017]
signaling whether both the actual frequency undershoots a
predefined maximum frequency and the actual attenuation undershoots
a predefined maximum attenuation.
[0018] According to a further aspect of the invention, there is
provided a device of the type mentioned at the outset, said device
having an oscillating circuit, a metallic part made of a first
material and a pulse generator for driving the oscillating circuit
with at least one pulse in order to generate a decaying oscillation
which is dependent on a distance between the metallic part and the
oscillating circuit and has an actual frequency and an actual
attenuation, and also having a signal transmitter which is designed
to signal when both the actual frequency undershoots a predefined
maximum frequency and the actual attenuation undershoots a
predefined maximum attenuation.
[0019] A special feature of the invention can be seen in the fact
that it is possible to protect against manipulation in a simple
manner despite the different characteristics of ferromagnetic and
non-ferromagnetic materials and mixtures of these materials.
[0020] The invention comprises comparing both the actual frequency
of the decaying oscillation with a predefined maximum frequency and
the actual attenuation with a predefined maximum attenuation. This
makes it possible to detect the required minimum distance between a
metal and the oscillating circuit, in particular the oscillating
circuit of a proximity sensor. Although the invention also allows
the type of approaching metal to be detected and also allows a
numerical value for the distance to be provided, the minimum
distance which acts as protection against manipulation can also be
monitored without specific knowledge of this information.
[0021] This is based on the insight that checking the actual
frequency and the actual attenuation makes it possible, in a simple
manner, to protect against manipulation both for ferromagnetic
metals and for non-ferromagnetic metals. This also applies to a
plurality of metals having a mixture of ferromagnetic and
non-ferromagnetic components. The principle of this approach is
explained in more detail using the exemplary embodiments.
[0022] In addition to the application in a guard door described
above, a corresponding method and a corresponding device may also
be advantageously used in a press. If the activation of a press is
intended to be possible only from a particular position, it is
possible to determine, in the manner described above, whether the
position required for operation has been reached. For example, it
is possible to detect whether an actuating cam which is assigned to
the drive of the press is at a particular position or is within a
particular range of angles. Only in this case will the press be
enabled for operation.
[0023] One refinement of the invention signals whether the actual
frequency undershoots a predefined minimum frequency and/or the
actual attenuation also undershoots a predefined minimum
attenuation.
[0024] In addition to the required protection against manipulation,
this also makes it possible to monitor whether the metallic part is
sufficiently close to the oscillating circuit. This means, for the
application of the guard door, that a closed guard door or an open
guard door can be detected, but manipulation by virtue of a metal
part being applied to the proximity sensor is detected at the same
time. The same applies to the application of the actuating cam, in
which case it is also possible to determine here whether the cam is
actually in the vicinity of the proximity sensor, or else an
overlying metal part is detected.
[0025] The signaling may be effected in different ways. It is
possible, on the one hand, to indicate the exceeding of the minimum
frequency and the minimum attenuation independently of the
undershooting of the maximum frequency and the maximum attenuation,
for example by transmitting two or three bits. However, combined
signaling is likewise possible. Then, an overall indication is
given of whether the actual frequency undershoots a predefined
maximum frequency, the actual attenuation undershoots a predefined
maximum attenuation and of whether the actual frequency exceeds a
predefined minimum frequency and/or the actual attenuation exceeds
a predefined minimum attenuation. Such a combined signal is
sufficient in many applications since, in order to avoid switching
on a system or to switch off a system, it may be sufficient to know
that one of the required safety conditions has been violated, but
the specific safety condition which has been violated may initially
be unimportant.
[0026] In another refinement of the invention which is also per se
an inventive development of the known method and the known device,
a data record is provided which represents a first correlation
between the frequency and the attenuation when the metallic part
approaches, and the signaling indicates whether the actual
frequency and the actual attenuation correspond to the first
correlation.
[0027] This embodiment, which is also functional without a
comparison with a maximum frequency and a maximum attenuation, is
based on the insight that metallic parts made of different
materials have different frequency/attenuation characteristics. For
example, the frequency of the oscillating circuit scarcely changes
if iron steel (ST37) approaches the oscillating circuit, whereas
the attenuation changes to a considerable extent when iron steel
approaches. If stainless steel (V2A) approaches, which is another
example, there is a similar change in the attenuation to that for
iron steel, while there is now additionally a change in the
frequency. Therefore, the frequency/attenuation characteristic of
stainless steel is considerably different from the characteristic
of iron steel. Copper (Cu) may finally be mentioned as a further
example which, although resulting in less attenuation when it
approaches the oscillating circuit, results in a greater frequency
change. The frequency/attenuation characteristic of copper thus
again differs from the characteristics of stainless steel and iron
steel.
[0028] Such a correlation between the frequency and the attenuation
as a metallic part made of a particular material approaches is
provided in a data record. In the simplest case, such a data record
comprises a frequency/attenuation data point or the description of
a straight line which can be used to determine different
frequency/attenuation points. However, more complex functions or
characteristic curves described using support/interpolation values
can also be used to describe the correlation between the frequency
and the attenuation. In this case, the data provided in the data
record can be determined by means of experiments and/or
computation.
[0029] When carrying out the method, if an object then approaches
the proximity sensor, an actual frequency and an actual attenuation
are determined for the oscillation of the oscillating circuit, and
it is signaled whether the actual frequency and the actual
attenuation correspond to the correlation stored in the data
record. If a correspondence is determined, this allows the
conclusion that the material whose characteristic is provided in
the data record is approaching the proximity sensor.
[0030] On account of the knowledge that the frequency/attenuation
characteristics of the different materials differ to a discernible
extent, this determination can be made, as far as possible,
independently of the distance. (Reliable differentiation is
difficult only in the case of a large distance. However, this is
not significant for the preferred applications since a large
distance indicates an unsafe state anyway, for example an open
guard door).
[0031] It is possible to check whether the actual frequency and the
actual attenuation correspond to the stored correlation in
different ways. For example, it is possible to define that a
2-tuple formed from the actual frequency and the actual attenuation
may differ from the provided correlation between the frequency and
the attenuation by, at most, a particular amount. If the
correlation is stored, for example, in the form of a straight line
or characteristic curve, the distance between the 2-tuple and the
straight line or the characteristic curve can be determined using
known mathematical methods. If the distance calculated undershoots
the defined maximum value, this is understood to mean that the
actual frequency and the actual attenuation correspond to the
correlation. If the distance is greater than the maximum value,
this means that they do not correspond to the correlation.
[0032] In another refinement of the invention, the data record
describes a distance between the metallic part and the oscillating
circuit, which distance is dependent on the frequency and
attenuation, and a distance value is determined on the basis of the
actual frequency and the actual attenuation.
[0033] This is based on the insight that a particular
frequency/attenuation value (2-tuple of frequency and attenuation)
also allows an immediate conclusion regarding the distance value of
the metallic part with respect to the proximity sensor. Although a
conclusion regarding the material of the approaching object can
also be drawn from the frequency/attenuation value, it is possible,
on account of the different frequency/attenuation characteristics,
to determine a distance value without having to evaluate material
information for this purpose. A distance value can therefore be
directly determined when the actual frequency and the actual
attenuation are known.
[0034] The distance value is determined in such a manner that the
corresponding distance is determined in the data record for a given
actual frequency and actual attenuation. This may again be carried
out, for example, in such a manner that that value of the data
record which has the smallest distance from the measured 2-tuple in
the mathematical sense is used.
[0035] As a further alternative, it is possible to determine the
two closest values and to interpolate or extrapolate the distance
value between these two values. This also makes it possible to
directly assign a distance value to a measured 2-tuple of the
actual frequency and the actual attenuation.
[0036] Another refinement of the invention signals whether the
distance value exceeds a minimum distance value and/or undershoots
a maximum distance value.
[0037] It is possible to protect against manipulation by taking
into account a minimum distance value, and/or a sufficient
proximity of the metallic part can be determined by taking into
account a maximum distance value.
[0038] In another refinement of the invention, the data record
represents a second correlation between the frequency and the
attenuation when a metallic part made of a second material that is
different from the first material is approaching the oscillating
circuit, and it is signaled whether the actual frequency and the
actual attenuation correspond to the second correlation.
[0039] The statements above accordingly apply to this embodiment.
However, storing a second correlation now makes it possible to
detect at least three states: [0040] a) an object which corresponds
to the first correlation and thus to the first material is
approaching; [0041] b) an object which corresponds to the second
correlation and thus to the second material is approaching; [0042]
c) an object which does not correspond to the first correlation or
the second correlation and thus does not correspond to the first
material or the second material is approaching.
[0043] Concerning the type of implementation, again, many
possibilities arise. The information determined can be respectively
signaled individually, can be signaled in groups, each group also
being able to contain logic combinations of different information,
or else can be output in the form of an overall signal. The overall
signal may indicate, for example, whether, in addition to complying
with the predefined minimum/maximum values, the condition that the
approaching object be composed of either the first material or the
second material has also been satisfied.
[0044] Alternatively, it is possible to output a signal at a first
level, for example when all conditions have been satisfied, whereas
a signal at a second level is output when at least one of the
conditions has not been satisfied. This is advantageous for safety
technology since it is then possible to signal in a simple manner
whether or not all safety-relevant conditions for operation of the
system have been satisfied.
[0045] In another refinement of the invention, it is signaled to
which of the correlations stored in the data record the actual
frequency and the actual attenuation correspond.
[0046] This makes it possible to additionally provide the
information relating to the material of which the object is
composed or the material which is predominantly contained in the
object which is approaching the proximity sensor.
[0047] In this case, the material can be determined by checking
which of the correlations stored in the data record the 2-tuple of
the actual frequency and the actual attenuation best corresponds
to. In this case, correspondence can be determined as described
above, for example the correlation from which the 2-tuple has the
smallest distance. Further queries, for example in order to
determine whether the 2-tuple exceeds a maximum permitted distance
from the closest correlation, may additionally be implemented, with
the result that fault handling is also possible.
[0048] In another refinement of the invention, the data record
stores at least one further correlation for a further material, and
it is signaled whether the actual frequency and the actual
attenuation correspond to a correlation from a particular group of
correlations.
[0049] This makes it possible to define, for example, a group of
materials which are intended to give rise to particular signaling.
For example, the correlations for iron steel, stainless steel,
aluminum (Al) and copper may thus be stored, wherein a group
consisting of iron steel and copper and a group consisting of
stainless steel and aluminum are defined. In the proposed
embodiment, it is then possible to signal, inter alia, whether the
2-tuple corresponds to the group of stainless steel and aluminum or
whether the material of an approaching object is stainless steel or
aluminum.
[0050] In another refinement of the invention, the method is
carried out twice, and a fault is signaled if there is a difference
in the respective resultant signaling.
[0051] In order to reduce the likelihood of a system being enabled
for operation even though the safety-relevant conditions have not
been satisfied as a result of a fault when carrying out the method
or as a result of a fault in the signaling device, it is
advantageous to secure the method and the signaling device using
redundancy. This may be achieved, for example, by running through
the method twice at short intervals of time.
[0052] However, it is preferred to carry out the method in two
different parts of the signaling device which are essentially of
the same design. In this case, it is assumed that the first
operation of carrying out the method (in the first part of the
device) and the second operation of carrying out the method (in the
second part of the device) must lead to the same result.
[0053] The result of the first signaling device and the result of
the second signaling device are then passed to a comparison unit
which compares the two result signals. If the result signals do not
match, a malfunction is assumed and a fault is signaled.
[0054] The fault may be indicated as a separate signal or a
combined signal. For example, a signal at a first level can be used
to indicate that all safety-relevant conditions have been satisfied
and that there is no fault, whereas a signal at a second level is
used to indicate that at least one of the safety-relevant
conditions has not been satisfied or that there is a fault. This
makes it possible to signal release or non-release of the system in
a simple manner.
[0055] It goes without saying that the abovementioned features and
the features still to be explained below can be used not only in
the respective combination stated but also in other combinations or
on their own without departing from the scope of the present
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0056] Exemplary embodiments of the invention are illustrated in
the drawing and are explained in more detail in the following
description. In the drawing:
[0057] FIG. 1 shows a set-up having a signaling device according to
the present invention;
[0058] FIG. 2 shows a two-channel signaling device which is
connected to a control unit;
[0059] FIG. 3 shows the sequence of a method according to the
invention in the representation of a flow diagram;
[0060] FIG. 4 shows frequency/attenuation characteristics with
associated distance values for different materials;
[0061] FIG. 5 shows the illustration according to FIG. 4 with the
minimum and maximum values depicted; and
[0062] FIG. 6 shows the illustration according to FIG. 4 with
dedicated permitted ranges.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0063] FIG. 1 uses the reference number 12 to denote an entire
set-up having the novel signaling device 10. In this case, the
set-up 12 comprises a power supply 14, a machine 16 and two
signaling devices 10 which are connected to a control unit 18. In
this case, the signaling devices 10 are constructed according to
the principle of an inductive proximity sensor. In general, it is
sufficient to use one signaling device 10.
[0064] The machine 16 can be switched on only when the two
signaling devices 10 signal a release. The signaling devices 10
indicate whether each of the metallic parts 22 which are arranged
on the guard door 20 are in a defined distance range with respect
to their respectively associated signaling device 10. In this case,
monitoring is carried out to ensure that the metallic parts 22
comply with a minimum distance with respect to the respective
signaling device 10 but at the same time also do not exceed a
particular maximum distance. Of course, a metallic guard door 20
may also itself perform the function of the metallic parts 22, with
the result that there is no need for separate parts 22.
[0065] If the two signaling devices 10 indicate that the guard door
20 has been closed correctly and thus that the machine 16 is
allowed to be started, the control unit 18 drives two contactors
24, 26 whose normally open contacts 28, 30 are arranged in the
connection between the power supply 14 and the machine 16. The
machine 16 can carry out the operation only when both contactors
24, 26 close their respective normally open contacts 28, 30.
[0066] In an alternative embodiment which is schematically
indicated here, a start button 19, which may be connected to the
control unit 18, also needs to be operated in order to switch on
the machine 16 when the guard door 20 is closed.
[0067] If one of the two signaling devices 10 no longer indicates
the release for the system 16, for example because the guard door
20 was opened during operation, the control unit 18 lets the
contactors 24, 26 drop. It is also conceivable to assign a
contactor 24, 26 to each of the signaling devices 10, with the
result that, when there is no release from one of the signaling
devices 10, the associated contactor 24, 26 deliberately drops. In
any case, the machine 16 thereby becomes deenergized.
[0068] FIG. 2 shows, in detail, one of the signaling devices 10
which is of redundant design in this case. The signaling device 10
has two modules 32 which are essentially structurally identical and
each have an oscillating circuit 34, a pulse generator 36 and a
signal transmitter 38. In this exemplary embodiment, each
oscillating circuit 34 has a capacitor and a coil. It is pointed
out that the pulse generator 36 is depicted separately from the
signal transmitter 38 for the sake of a better overall view but the
pulse generator 36 may also be integrated with the signal
transmitter 38.
[0069] The outputs from the signal transmitters 38 are passed to a
comparison unit 40 which is designed to compare the signals from
the signal transmitters 38 and to signal a fault if the signals do
not match. The output signal from the comparison unit 40 is passed
to the control unit 18 and is evaluated therein. The signaling
device 10 is also assigned a metallic part 22 made of a first
material. The signal transmitters 38 are each assigned a memory 42
which stores correlations between the frequency and the attenuation
for different materials when a metallic part 22 approaches the
signaling device 10. The dashed line indicates that the memory 42
is optional.
[0070] It is pointed out that the signaling device 10 may also be
constructed with only one oscillating circuit 34, which is then
tapped off by the two comparison units 40, in particular in order
to save space.
[0071] The method of operation of the signaling device 10 shall
first of all be explained using the implementation of protection
against manipulation. Since the modules 32 have the same
functionality, the explanation now refers to one of the modules
32.
[0072] The pulse generator 36 sends a pulse to the oscillating
circuit 34. The pulse produces a decaying oscillation having an
actual frequency and an actual attenuation in the oscillating
circuit 34. In this case, the actual frequency and the actual
attenuation are dependent both on the distance d between the
metallic part 22 and the signaling device 10 and on the material of
the metallic part 22.
[0073] The actual frequency and the actual attenuation are
determined and are compared with a predefined maximum frequency and
a predefined maximum attenuation. The signal transmitter 38 may
provide a corresponding measuring and comparison device (not
illustrated in any more detail here) for this purpose.
[0074] If both the actual frequency undershoots the maximum
frequency and the actual attenuation undershoots the maximum
attenuation, the signal transmitter 38 generates a release signal
which is passed to the comparison unit 40.
[0075] If the comparison unit 40 receives a release signal from
both modules 32, it sends a release signal to the control unit 18.
If one of the two signal transmitters 38 does not send a release
signal, the comparison unit 40 does not send a release signal
either. This behavior thus simultaneously safeguards the fault
situation in case the signal transmitters 38 send different
signals.
[0076] In the exemplary embodiment shown here, the signaling device
10 additionally monitors whether the actual frequency exceeds a
predefined minimum frequency and the actual attenuation exceeds a
predefined minimum attenuation. If this is not the case, the signal
transmitter 38 does not send a release signal.
[0077] Further monitoring options which result from including the
optional memory 42 are explained in even more detail below.
[0078] FIG. 3 shows the sequence of a method according to the
invention. In this case, an oscillating circuit 34 is provided in a
first step 50 and a metallic part 22 made of a first material is
provided in a second step 52. In a step 54, the oscillating circuit
34 is then driven with a pulse, thereby producing a decaying
oscillation which is dependent on a distance d between the metallic
part 22 and the oscillating circuit 34 and has an actual frequency
and an actual attenuation. A step 56 then signals whether both the
actual frequency undershoots a predefined maximum frequency and the
actual attenuation undershoots a predefined maximum attenuation.
The method is then continued again with step 54.
[0079] FIG. 4 illustrates the fundamental insight used by the
invention for secure distance monitoring. A decay time of the
oscillation generated in the oscillating circuit 34 is illustrated
along the abscissa. The decay time is the time which passes, after
the pulse has been sent to the oscillating circuit 34, until the
maximum amplitude of the oscillation has decreased to a particular
percentage, for example 10%, on account of the electrical losses.
The decay time is therefore a measure of the attenuation. The
shorter the decay time, the greater the attenuation, and the longer
the decay time, the smaller the attenuation.
[0080] The frequency of the oscillation in the oscillating circuit
34 is plotted along the ordinate.
[0081] FIG. 4 illustrates correlations between the frequency and
the attenuation (decay time) for iron steel (ST37), stainless steel
(V2A), aluminum (Al) and copper (Cu). A characteristic curve
representation with support/interpolation values was selected in
this case, but the characteristic curves may also be approximated
by means of simple functions, for example a linear function or
exponential function.
[0082] By way of example, it can be seen from FIG. 4 that, for an
actual frequency of approximately 72 kHz and an actual attenuation
represented by a decay time of approximately 200 .mu.s, it is
possible to infer stainless steel at a distance of 2 mm or, for a
frequency of approximately 74 kHz and an actual attenuation
represented by a decay time of approximately 475 .mu.s, it is
possible to infer a metallic part 22 made of copper at a distance
of 1 mm. It is thus possible to infer the material of the metallic
part 22 and/or the distance d between the latter and the
oscillating circuit 34 from a 2-tuple of the actual frequency and
the actual attenuation.
[0083] FIG. 5 shows an embodiment which affords simple and reliable
protection against manipulation even without a correlation between
the frequency and the attenuation, which is stored in a data
record.
[0084] In order to illustrate the protection against manipulation,
reference is first of all made to the maximum frequency f.sub.MAX
of approximately 73.5 kHz and to the maximum attenuation Z.sub.MAX
represented by a minimum decay time of approximately 175 .mu.s. The
range which exceeds the maximum frequency f.sub.MAX is illustrated
using double hatching. The range which exceeds the maximum
attenuation Z.sub.MAX is illustrated using thin single
hatching.
[0085] Since both the actual frequency and the actual attenuation
of the oscillation in the oscillating circuit 34 are checked, this
means that a release signal is not generated if any desired
material of the possible materials is at a distance of less than 1
mm from the oscillating circuit 34.
[0086] It is expressly stated at this point that the diagram for
particular 2-tuples of the actual frequency and the actual
attenuation shows a correspondence to a material and a distance
value but that there is no need to obtain or evaluate this
information in order to protect against manipulation. Rather,
evaluation of the actual frequency and the actual attenuation with
respect to the respective maximum value is sufficient without the
need to take into account the associated material or distance.
[0087] A special feature of the proposed protection against
manipulation is that it is simple to implement and nevertheless
works reliably with a plurality of different materials, in
particular ferromagnetic and non-ferromagnetic materials.
[0088] This is also a flexible solution since the range detected by
the protection against manipulation can be varied by simply varying
the maximum frequency f.sub.MAX and the maximum attenuation
Z.sub.MAX. If, for example, the maximum frequency f.sub.MAX is
predefined at approximately 72.5 kHz and the maximum attenuation
Z.sub.MAX is predefined by a decay time of approximately 190 .mu.s,
a minimum distance of approximately 2 mm is defined for all
possible metals.
[0089] A maximum distance can be monitored in a similar manner. For
this purpose, a minimum attenuation Z.sub.MIN is predefined in the
form of a maximum decay time. The range excluded by this
stipulation is illustrated using thick single hatching. If
necessary, a minimum frequency f.sub.MIN may also be defined. The
resultant range is illustrated using the dashed double
hatching.
[0090] It goes without saying, however, that a statement relating
to the material and/or the distance can also be made in this
embodiment if the memory 42 stores the corresponding correlations.
However, it is pointed out again that both the minimum distance
needed to protect against manipulation and the maximum permissible
distance can be implemented solely by comparing the actual
frequency and the actual attenuation with respectively associated
minimum/maximum values.
[0091] FIG. 6 shows an embodiment in which only specific materials
with specific distances are allowed in order to obtain a release
signal. In this embodiment, the correlations for selected materials
are stored in a data record, particular ranges being defined as
permissible (not hatched) and other ranges being defined as
impermissible (hatched) by means of an appropriate checking
algorithm. The settings were made in this example in such a manner
that a release signal is issued when a metallic part 22 made of
stainless steel or copper has approached in the range of
approximately 1 mm to 5 mm, whereas a release signal is not sent in
other distance ranges or for other materials. It is also possible
to respectively specify the material which has been detected and
the distance value on the basis of the data stored in the data
record.
[0092] However, on account of the known correlation between the
frequency and the attenuation, which can be determined by
computation and/or experiments, it is also possible to dispense
with storing material information and/or distance information since
a query in order to determine whether a particular 2-tuple of the
actual frequency and the actual attenuation is in a permissible
range is sufficient.
[0093] It goes without saying that more complex forms may also be
used to describe the permissible and impermissible ranges, or only
sections of characteristic curves are stored. The ranges may also
be described, in particular, as functions by means of formulas and
may be provided with a tolerance band.
[0094] Overall, the invention shows a simple and reliable concept
for constructing and operating a signaling device with protection
against manipulation. Furthermore, the invention enables a
comprehensive definition of permissible and impermissible states,
that is to say those states in which a release signal is or is not
generated. In this case, it is possible to define the same minimum
distance for different materials in a simple manner or else to
provide for a release signal to be generated when there is no
distance value for particular materials. It is possible to both
explicitly include and explicitly exclude particular materials
in/from the detection. In addition to the simple implementation of
the fundamental idea, the invention therefore also provides a basis
for implementing demanding requirements.
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